Paper by MAURO CONI from ISCR 8th 1998 Lisbon Portugal
As a general rule procedures for the sizing of road superstructures only refer to static stress. Possible dynamic effects induced by loads are introduced by amplifying their intensity or by considering the dynamic modulus of the materials. Under variable stress in time, the real behaviour of a superstructure is remarkably complex due to the heterogeneity of the different layers. It has generally been observed that dynamic deflections corresponding to a few load frequencies in the vicinity of the resonance frequencies of the superstructure are amplified. The peak width, central frequency and band width of each resonance closely depend on the geometric, rheological and dissipative characteristics of the materials used. Resonance is at its maximum when it is close to the first resonance frequency. Under these conditions the dynamic static subsidence ratio, öd/s, reaches values in the vicinity of 2.7--3.2. Moreover, this ratio increases at points far from the stress axis and tends to unity on tT l ncreasing the depth of the superstructure. he article first presents a iiiudal analysis and then a harmonic analysis in a finite element model (FEM) of a oncrete superstructure. From the modal analysis it has been possible to determine the superstructure's resonance frequencies in the 1-100 Hz range. Harmonic analyses have been referred to different stress conditions and load positions. Particularly, it has been possible to assess the dissipative ability of the system by measuring modal damping due to the change in the thickness and mechanical characteristics of the materials. A parametric study of the harmonic analysis was developed by modifying the energy dissipation coefficient value. Moreover, the amplification of the stress state of a given strain applied dynamically with increasing pulsation was assessed. Finally, we investigated the benefit due to the use of highly damping materials in the superficial layer, such as bituminous concrete, that could improve significantly the generation and transmission of vibrations. Particular attention was given to the geometry, the materials in terms of stiffness, viscosity and damping, and to the inertial effects of the structure, in so much as they contribute to the vibration state. Finally a few useful remarks even from a design point of view, that are inherent in the dynamic response of rigid superstructures, are made. A comparison with the traditional static method shows that it is not possible to prescind from a dynamic assessment when vibrational effects become preponderanL
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